In general, strength of conventional steel wires, for example bridge wires, has not exceeded the upper limit of 1600 MPa. In accordance with recent trend for building large bridges, a demand for high strength wires has increased. Such a high strength is also demanded for other steel wires such as PC steel wires.
In general, to provide high strength wires, high carbon hard wires are produced by subjecting hot-rolled wire rods to a patenting treatment, as required, and thereafter the wire rods are drawn, thereby obtaining steel wires having a predetermined diameter. By such a treatment, steel wires are required to have a strength of 1600 MPa or more and a sufficient ductility which is, for example, evaluated on the basis of a reduction of area after breaking.
In order to satisfy the above-described demands, attempts have been made to increase the drawing workability of the high carbon wire rod by controlling segregations or microstructures or by adding a particular element.
A reduction of area of patenting wire rods depends on a grain size of austenite. Specifically, the reduction of area can be improved by refining the grain size of austenite. Thus, attempts have been made to decrease the austenite grain size by using nitrides or carbides of Nb, Ti, B and the like as pinning particles.
A wire rod has been suggested in which as a chemical composition, one or more elements selected from the group consisting of 0.01 to 0.1 wt % of Nb, 0.05 to 0.1 wt % of Zr and 0.02 to 0.5 wt % of Mo, in mass percent, are added to a high carbon wire rod (e.g., Patent Document 1: Japanese Patent No. 2609387).
Another wire rod has been suggested in which NbC is contained in a high carbon wire rod to refine a grain size of austenite (e.g., Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2001-131697),